Material processing apparatus, such as lasers and plasma arc torches, are widely used in the cutting, welding, and heat treating of metallic materials. A laser-based apparatus generally includes a nozzle through which a gas stream and laser beam pass to interact with a workpiece. Both the beam and the gas stream exit the nozzle through an orifice and impinge on a target area of the workpiece. The laser beam heats the workpiece. The resulting heating of the workpiece, combined with any chemical reaction between the gas and workpiece material, serves to heat, liquefy and/or vaporize a selected area of workpiece, depending on the focal point and energy level of the beam. This action allows the operator to cut or otherwise modify the workpiece.
Similarly, a plasma arc torch generally includes a cathode block with an electrode mounted therein, a nozzle with a central exit orifice mounted within a torch body, electrical connections, passages for cooling and arc control fluids, a swirl ring to control fluid flow patterns in the plasma chamber formed between the electrode and nozzle, and a power supply. The torch produces a plasma arc, which is a constricted ionized jet of a plasma gas with high temperature and high momentum that exits through the nozzle orifice and impinges on the workpiece. Gases used in the torch can be non-reactive (e.g., argon or nitrogen), or reactive (e.g., oxygen or air).
It is generally desirable that the results of any material processing be of high quality. For example, the edges of the cut kerf produced by laser and plasma cutting should be dross-free, smooth, straight and uniform. Edge irregularities caused by, for example, uneven heating of the workpiece by the laser, excessive chemical reactions between the assist gas and workpiece, or incomplete removal of cutting debris, should be minimized.
Presently, the operation of CNC-controlled plasma arc or laser cutting systems typically requires several manual parameter adjustments to achieve workpiece processing results of desired quality. Consequently, users typically choose conservative values of process parameters to ensure process reliability over a wide range of operating conditions. The tradeoff often results in an accompanying decrease in material processing productivity (e.g., due to a reduced cutting speed in laser cutting). For more aggressive process parameters to be used, a reliable and automated means of monitoring the cutting process is necessary, which could alert the user to degradation in the quality of the cut in real time. Such a system could also be required to adjust to changes in operating conditions to maintain optimal process performance, i.e., good cut quality and maximum productivity.